Work stations, personal computers and portable computers require disc drives that provide a large amount of data storage within a minimum physical area. Generally, disc drives operate by positioning a transducer (or read/write head) over respective tracks on a rotating magnetic storage disc. Positioning of the transducer over the tracks is accomplished by an actuator coupled to control electronics, to control the positioning of the actuator and the read/write functions of the transducer.
Greater demands are being placed on disc drives by the use of multi-user and/or multi-tasking operating systems, work stations which provide an operating environment requiring the transfer of large amounts of data to/from a hard disc and/or large numbers of disc accesses to support large application programs or multiple users, the present popularity of notebook and laptop computers and the continuing trend toward higher performance microprocessors. Also, systems require hard disc drives having high capacity storage capability, while occupying a minimal amount of space within the host system. In order to accommodate these demands, there is a need for smaller hard disc drives which, at the same time, have increased storage capacity.
One measure of determining the storage capacity of a disc drive is the flying height of the transducer above the rotating disc. In conventional hard disc drives, once the hard disc achieves a certain angular velocity after startup of the drive, a cushion of air above the surface of the hard disc forces the transducer up off the surface of the disc by a very small amount to thereby achieve a flying height. Flying the transducer closer to the disc surface allows for higher data bit a density (i.e., the number of data bits per square inch on the disc surface). Consequently, there has been an industry-wide push to decrease the height at which transducers are maintained over the disc surface without actually contacting the disc surface.
When a transducer flies over a rotating disc, the flying height tends to fluctuate slightly above and below the normal flying height because the disc surface is not flat. At lower flying heights, a variation in the fly height may cause the transducer to randomly contact the disc surface. This situation is referred to as intermittent contact. Repeated intermittent contact between the transducer and the disc surface can damage the transducer and/or the disc, and may cause drive failures in an extremely short period of time.
Special attention must be paid to the mechanism used to clamp the disc or discs within the drive. Disc distortion caused by conventional clamping mechanisms has become a significant problem, particularly near the inner diameter of the disc.
In conventional disc drives, the disc is provided on a cylindrical hub of a spindle motor. A clamp is provided on top of the hub, having a larger radius than that of the hub such that an outer circumferential portion of the clamp is in contact with the disc. A plurality of screws fit through holes around the circumference of the clamp, and into threaded bores in the hub. When the screws are tightened, the force applied by the screws is transferred to the outer circumferential portion of the clamp, which contacts the disc in order to secure the disc or discs to the spindle motor hub. The discs must be secured under a considerable force in order to prevent any slippage of one or more discs in the presence of mechanical shocks. Even very slight slippage of a disc within a drive could result in mechanical off tracking of the transducer which could result in data transfer errors or servo failure.
Since an individual screw can only deliver a certain amount of clamping force, the total member of screws required to secure the discs to the spindle motor can he determined for a given application once the size of the screws is defined.
Ideally, the force exerted by the disc clamp at the circular line of contact defined between the clamp and disc should be uniform around the entire line of contact. However, the force is stronger at points around the line of contact located radially outward from the screws and weaker in between the screws. The fluctuation of the clamping force around the line of contact will cause circumferential waviness (or distortion) of the disc, especially near the inner diameter thereof.
One way to reduce disc waviness, when the disc is clamped to a spindle motor, is to use a greater number of screws than required by the particular clamping force. The greater the number of screws, the more uniform the clamping force. However, increasing the number of screws used has a negative affect on disc drive manufacturability because of the need to repeatedly tighten the screws in a particular sequence in order to provide a uniform clamping force over the inner diameter of the disc.
Another way to reduce disc waviness is to provide a stiffening ring between the screws, used to hold the disc(s) in contact with the spindle motor, and the line of contact as disclosed in U.S. Pat. No. 5,528,434, assigned to the assignee of the present invention. The stress concentration around each screw dissipates after application of the stiffening ring. Thus, the clamping force can be distributed evenly around the line of contact. A drawback of using a stiffening ring to reduce disc waviness is that the screws need to be located toward the inner diameter of the disc clamp in order to provide adequate space for the stiffening ring. As a result, the top portion of the spindle motor hub must be made thicker in order to provide enough threaded area to accept the screws. Consequently, less space is available for laminations inside the spindle, thereby reducing spindle motor power.